1. Field of the Invention
This invention relates to a marine fender for buffering shocks to vessels and offshore structures in harbors or on the sea.
2. Description of the Prior Art
In the conventional marine fender a, as shown in FIGS. 1 to 4, a shock receiving portion b of a square plate type is integrally united at the outer periphery with an upper peripheral edge of a support portion c of a hollow pyramid frustum, and this support portion c has an approximately equal thickness and a certain slant as shown in FIG. 2.
In berthing the vessel, therefore, the dynamic phenomenon of the marine fender a applied to compressive force is as follows.
When the thicknesses of the support flat portion d and the support corner portion e are td and te, respectively, as shown in FIG. 3, geometrical moments of inertia Id and Ie in the portions d and e are proportional to the cubes of the thicknesses td and te, respectively.
Now, when the compressive force P is applied to the marine fender a, if the compressive force P is relatively small, both the shock receiving portion b and the support portion c cause an elastic deformation, while if the compressive force P is large and exceeds a buckling load, the support portion c causes a buckling deformation. Moreover, the buckling load Pc is represented by the following equation: EQU Pc=k(EI/l.sup.2) (1)
wherein k is a proportionality factor, l is a longitudinal overall length of the support portion and E is an elastic coefficient, and hence is proportional to the geometrical moment of inertia I. On the other hand, the relation between the thickness td of the support flat portion and the thickness te of the support corner portion is te&gt;td as apparent from FIG. 3, so that the geometrical moment of inertia is Ie&gt;&gt;Id. As a result, the support corner portion e does not materially cause buckling deformation as compared with the support flat portion d. That is, only the support flat portion d causes the buckling deformation.
In case of causing the buckling deformation, the support corner portion e retains a right angle between the adjoining support flat portions because of the resistance to bending deformation along a plane parallel with the shock receiving surface or the bending rigidity in a direction parallel with the shock receiving surface. As shown in FIG. 4, therefore, the opposed support flat portions d.sub.1 bulge outward, while the opposed support flat portions d.sub.2 dent inward. That is, the support corner portion e having a large buckling load can not a great influence on the resistance causing no buckling of the support portion c as a whole, so that the conventional marine fender a rapidly changes from a compression elastic deformation region having a high reaction force into a buckling deformation region having a low reaction force, whereby the shock absorbing energy against the vessel becomes reduced.